ada: Eliminate dead code

We never use the cgd that we get from the XPT_GDEV_TYPE call. Prior to
9a6844d55fe33 we used it to determine if READ AHEAD or WRITE CACHING was
supported. However, all that information was moved into adasetflags so
we no longer need to this since it's cached in the softc and updated
with the IDENTIFY data changes automatically.

Sponsored by: Netflix
Reviewed by: mav
Differential Revision: https://reviews.freebsd.org/D35039